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In Reply to: RE: Are there horn profiles or geometries that work better than others? posted by Edp on April 10, 2024 at 09:33:19
Hi
There are (at least) two things going on, if you want / need constant directivity (where the highs extend to the edges of the lower frequency pattern), then you need to use a horn who's walls are more or less straight and obeys some other horn related guide lines.
The reason is for both the acoustic transformation and pattern control, the locations inside the horn that are in control all change with frequency, moving towards the throat with increasing frequency so a curved wall horn becomes narrower as the frequency climbs.
With the right driver and horn, this can have flat or pretty flat response. However, the power response of all hf horn drivers falls off around -6dB /oct 2 to 4Khz and if you have a CD horn, you get the shape of the power response which needs eq to be flat.
In a room, these two sound very different when off axis.
So far as two or more drivers combining into one source either inside (or outside) of a horn, they must be less than 1/4 wavelength apart at the highest frequency they interact.
This IS part of the stereo image. With separate sources, there is some distance one must be before the sources combine subjectively into one. However, "sounds like" isn't the whole story. With your eyes closed, listening to one speaker it's easy to point at where it is BUT you can also triangulate how far away it is thanks in part to the still separate radiations.
A single small source at significant distance has no distance clues to triangulate on, what reaches the right and left ears is identical. All you have to go on is the hf roll off and reflections as clues, It is the difference from one ear to another that allow triangulation in depth.
The cool / unexpected part developing these horns, about a single simple radiation is there is much less information radiated by the speaker about how far away it is and so with a good recording, the sources are not part of the stereo image.
This is something anyone can hear too.
Obtain and mount a pair of these drivers on a flat baffle at least 18 in square.
https://faitalpro.com/en/products/LF_Loudspeakers/product_details/index.php?id=401000160
Cover the baffle with 1/2 inch foam (super77), front mount the driver through the foam. Figure out a small sealed back box and set these up about 6 or 8 feet away pointed at you in normal stereo.
These drivers are an amazing little full range driver, small enough to radiate simply up to pretty high, no crossover phase and while they can't do low bass, they do a good job fading behind a phantom stereo image.
WE can hear things how sound is radiated in 3d, we measure in 1d even with the Klipple machine.
Tom
Follow Ups:
Bunch of DIY folks got in fury of making sets of those as it was purported to be a FOSTEX unit. It wasn't but bit of fun anyway.
Was doing a number of Open Baffle studie also at that time,, took that concept up to some Dynaudio 9" on 3 by 5 foot baffles. Not over great but still did single source bit of stereo illusion.
Bent your ear enough for now, but possibly in future can get some " lens" history and info.
No need to wait, a lens is easy to explain if one thinks in terms of time.
Imagine a convex lens (thick at the center, thin at the edges), the velocity of light in the glass is slightly less than in air and so if one imagines a flat plane of photons entering the glass (an optical plane wave) , the thicker the glass, the greater that photon is delayed relative to the outer edges.
The light that exits the glass that entered as a flat plane is now a converging wavefront (coming to a focal point) with the "F" number being the ratio of the diameter to focal length..
With Sound on the other hand the velocity of sound in air only changes with temperature so using different materials to cause a delay is out but one can use a longer / shorter path lengths for that purpose.
Consider the potato masher lens or a slanted plate lens. Both of these deal with a horn that has a very narrow pattern up high, the lens progressively delays the edges vs the center and this produces a more curved wavefront (wider dispersion angle) at the exit.
The Paraline is a reversed situation the driver radiation is radial 360 degrees but easier to imagine if one had say 20 equal length tubes attached to the driver exit who's exit ends are lined up in the straight line. This way the sound radiates from all the tubes at the same instant and arranged in a "line". Now, imagine that there are no tubes and it is the equal distribution of pressure that causes the distribution.
Hope that helps.
Tom
Appreciate the insight and explanation of the mechanisms involved.
Until you mentioned it I didn't think of horn lens like the JBL Hartsfield (2390 ?) relating to the examples of a lens applied to a direct radiator. Helps with grasping with mechanisms involved.
In his last few years Brian Cheney of VMPS was committing a number of his line source units with a lens. I never fully understood the mechanism he was working on, but the units he applied this method definitely "worked" as there was a difference with over without lens
Left no lens. Right with lens
In the link added for Graham Audio, I'm not sure, but it appears the vintage approach of slotting bass units is more of a directivity influence than what you describe for the lens
It's always an eye opener when I'm confronted with a design that "violates conventional guidelines" but actually works better.Possibly one day I'll stumble into an audio room and the Paraline can perform its eye opener. Did that with the Unity way back machine.
Edits: 04/12/24
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